Techniques used to determine the programs or other content to which audience members have been exposed are intended to gather such data at the audience members' locations. Various systems have been proposed for this purpose. In one variant, a stationary device is positioned near a television, radio, computer, or the like, in order to monitor media data at audience locations.
Another variant proposes the use of a portable device to be carried about by an audience member in order to gather data regarding the programs and other content to which the audience member has been exposed.
These devices obtain the signals to be monitored either through a direct electrical connection, or by means of a sensor such as a microphone, light-sensitive device, capacitive pickup or magnetic sensor. Typically the device either detects the presence of an ancillary code in the media data or else extracts a signature therefrom for pattern matching, and stores the code or signature for subsequent processing at a remote location. In order to produce audience surveys which are statistically reliable, it is necessary to engage a relatively large number of survey participants, so that it is likewise necessary to supply a relatively large number of monitoring devices, such as stationary or portable devices. It is, therefore, desirable to minimize the complexity of such devices in order to minimize their cost.